1. Field of the Invention
The present invention relates to a heat fixing device which is mounted in an image forming apparatus, such as a copier, a printer or a facsimile, includes a heating target member which is heated by induction heating, and fixes a toner image on a sheet by the heating target member.
2. Description of the Related Art
As a fixing device used for an image forming apparatus such as a electro photographic copier or printer, there is a device in which a sheet is inserted into a nip formed between a pair of rollers including a heat roller and a pressure roller or between a pair of a heat belt and a pressure roller, and a toner image is heated, pressed and fixed. In such a heating type fixing device, in order to realize speeding up of process speed, there is an induction heat fixing device in which a heat roller or a heating belt is heated by an induction heating system. In the induction heat fixing device, an eddy-current is generated in a metal conductive layer by a magnetic field which is generated by supplying specified power to an induction current generation coil. The metal conductive layer is instantaneously heated by this eddy-current, and for example, the heat roller is heated.
As one of such induction heat fixing devices, there is a device in which a very thin metal conductive layer is disposed near the surface of a heat roller or a heat belt, and an induction current generation coil is disposed outside the heat roller or the heat belt so as to be opposite thereto. In the device as stated above, since the heat capacity of the metal conductive layer is small and the metal conductive layer is disposed near the surface, the speed of heating of the surface of the heat roller or the heat belt can be further increased. On the other hand, since the metal conductive layer near the surface is very thin, there is a fear that it is damaged by the contact of a peel pawl or the like.
Thus, there is developed a device in which after fixing, a sheet paper is peeled off from a heat roller without using a peel pawl. For example, the metal conductive layer of the surface of the heat roller is supported by an elastic layer, so that the surface of the heat roller has elasticity, and the hardness of the surface of the press roller is made higher than the heat roller. By this, the curve of a section of a nip between the heat roller and the press roller is formed into such a curve that a sheet paper is predisposed to peel off from the heat roller. That is, although the peel pawl is not used, the surface of the heat roller is made to have elasticity, so that the sheet paper is peeled off from the heat roller by using the toughness of the sheet paper and the curl direction of the sheet paper.
However, in the heat roller or the heat belt which includes the metal conductive layer supported by the elastic layer and has elasticity in the surface, the right and left balance becomes unstable at the time of rotation. Thus, the heat roller or the heat belt is liable to laterally shift at the time of rotation.
For example, as shown in the Prior Art of FIG. 1, when a heat roller 100 laterally shifts and comes in contact with an edge 102a of a press roller 102, there is a fear that a crack 100b occurs in a thin metal conductive roller near the surface of the heat roller 100. Alternatively, when the heat roller 100 laterally shifts, a stress occurs in an elastic layer intervening between the metal conductive layer and a cored bar, and when it is a sponge or the like, there is a fear that a contact portion with the cored bar is broken.
Thus, for example, JP-A-2005-158639 or JP-A-2001-6868 discloses a flange to regulate the shift of a fixing film having a metal layer at the time of running.
However, in these devices of the relater art, although the shift of the fixing film is regulated, interference of the edge of the pressure roller is received due to its structure. Thus, there is a fear that by long-term use, a crack occurs in the metal layer by the contact with the edge of the press roller, and prolongation of the life of the heat roller is prevented.
Then, in a heat fixing device including a heating target member which includes a metal conductive layer supported by an elastic layer, and a pressure member, there is desired the heat fixing device which prevents a crack of the metal conductive layer of the heating target member, and prevents destruction of the elastic layer, so that the life of the heating target member is prolonged.